In the information age, the demand for data networks of higher and higher data capacities, at lower and lower costs is constantly increasing. This demand is fueled by many different factors, such as the tremendous growth of the Internet and the World Wide Web. The increasing number of on-line users of the Internet and the World Wide Web has greatly increased the demand for bandwidth. For example, Internet video clips require a large amount of data transfer bandwidth.
Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. Optical fiber offers much higher bandwidths than copper cable and is less susceptible to various types of electromagnetic interferences and other undesirable effects. As a result, optical fiber is the preferred medium for transmission of data at high data rates and over long distances.
In optical fiber communication systems, data is transmitted as light energy over optical fibers. The data is modulated on an optical light beam with an optical modulator. Optical modulators modulate the amplitude or the phase of the optical light beam. Direct optical modulators modulate the optical wave as it is generated at the source. External optical modulators modulate the optical wave after it has been generated by an optical source.
One type of external modulator is an electro-optic interferometric modulator, such as a Mach-Zehnder interferometric (MZI) modulator, that is formed on a X-cut or Z-cut lithium niobate substrate. A MZI modulator is a dual waveguide device that is well known in the art. In operation, an electromagnetic signal, such as a RF or microwave signal, interacts with an optical signal in one of the waveguides over a predetermined distance that is known as the interaction distance. The RF signal propagates in a coplanar waveguide (CPW) mode.
Typical high-speed electro-optical external modulators use a traveling-wave electrode structure to apply the RF signal. Such modulators have a RF transmission line in the vicinity of the optical waveguide. The RF signal and the optical signal co-propagate over an interaction distance, thereby acquiring the required optical modulation. The bandwidth of such structures is limited by a phenomenon known as “walk off,” which occurs when an electrical signal and an optical signal propagate with different velocities or group velocities.
A number of solutions have been suggested to limit “walk off” or to match the velocity of the optical signal to the velocity of the RF signal. One method of velocity matching the RF signal to the optical signal is to include a buffer layer on the top surface of the substrate that increases the propagation velocity of the RF signal to a velocity that is closer to the propagation velocity of the optical signal. Another method of reducing velocity mismatch between the RF signal and the optical signal is to decrease the interaction distance. Decreasing the interaction distance, however, requires an increase in the electric field that is required to obtain a suitable phase shift in the optical signal.
A method of reducing velocity mismatch between the microwave modulation signal and the optical signal propagating in the waveguide includes providing a buffer layer that has approximately the same effective dielectric constant as the optical waveguide and also introducing electrically floating electrodes between RF electrodes and the substrate to maximize the electric field across the waveguide.
However, such a structure may induce undesired longitudinal current in the ground electrodes that are electromagnetically coupled to the electrically floating electrodes. This undesired longitudinal current can negatively impact the performance of the modulator. For example, the undesired longitudinal current can result in coupled modes being created in the ground electrodes and in the electrically floating electrodes. The undesired longitudinal current can also result in conversion of the CPW mode to higher order modes in the ground electrodes and in the electrically floating electrodes. This modal coupling and modal conversion can lead to high frequency loss in the substrate, which can degrade modulator performance at high frequencies.